FIG. 9 is a block diagram illustrating the structure of an ordinary signal processing circuit in a single-chip image sensing apparatus having a primary-colors filter of the kind shown in FIG. 8. In this specification, red, green and blue signals will be abbreviated to R, G, B signals, respectively. An output signal from an image sensing device 901 is subjected to black balance and white balance adjustments in an OB circuit 902 and WB circuit 903, respectively. An adaptive interpolation circuit 904 performs adaptive interpolation processing for every color signal in such a manner that a signal will be present at all pixel positions. Generally, in case of a camera having a primary-colors checkered filter of the kind shown in FIG. 8, vertical or horizontal stripes are discriminated by detecting signal correlation above, below and to the left and right of a pixel to undergo interpolation. If a vertical stripe is discriminated, interpolation is performed from the upper and lower signals. If a horizontal stripe is discriminated, then interpolation is performed from the left and right pixels. Such adaptive interpolation is well known and the details thereof will not be described here.
The output of the adaptive interpolation circuit 904 is converted to color difference signals (R-Y, B-Y) by a color converting circuit 905, and false colors in the saturated region are removed by a CSUP (Chroma Suppress) circuit 906. Meanwhile, the output of the adaptive interpolation circuit 904 is converted to a luminance signal by a luminance signal creating circuit 907 and edge emphasis is applied by an edge emphasizing circuit (APC) 908. The color difference signals from the CSUP circuit 906 and an output signal Yh from the edge emphasizing circuit 908 are converted to an RGB signal by color converting circuit 909. A tone correction that conforms to the output apparatus is applied by a gamma circuit 910. The output signal of the gamma circuit 910 is converted to a YUV circuit by a color converting circuit 911.
FIG. 10 is a block diagram illustrating the details of the edge emphasizing circuit 908 shown in FIG. 9. In the case of an image sensing apparatus equipped with a single-chip primary-colors checkered filter of the kind shown in FIG. 8, the frequency band of the RB signal is only half that of the G signal. In general, therefore, only the G signal is employed in high-frequency-signal detection used in edge emphasis.
The operation of the edge emphasizing circuit 908 will be described with reference to FIG. 10. A signal Yg obtained by adaptive interpolation in the adaptive interpolation circuit 904 using the G signal is supplied from the luminance signal creating circuit 907 to bandpass filter circuits 1002 to 1004. The bandpass filter circuit 1002 for detecting high-frequency components in the horizontal direction (HBPF) detects an edge in the horizontal direction from Yg and outputs this as an edge signal. The edge signal is subjected to amplitude adjustment in a PP gain (peak-to-peak correct gain) circuit 1005, and noise components are removed by a BC (Base Clip) circuit 1008. Similarly, edge signals in the vertical direction (VBPF) and diagonal direction (DBPF) also are detected. The edge signals are added by adders 1011, 1012 and a gain adjustment is applied in a gain circuit 1013 to obtain a signal Yapc.
Meanwhile, the luminance signal creating circuit 907 generates a main luminance signal Ysig using Yr (R signal), Yg (G signal) and Yb (B signal) obtained by adaptive interpolation in the adaptive interpolation circuit 904. The main luminance signal Ysig is calculated in accordance with the following equation:Ysig=0.3×Yr+0.59×Yg+0.11×Yb  (1)
Finally, the main luminance signal (Ysig) and the output signal (Yapc) from the gain circuit are added by an adder 1014 and the sum is output as Yh.
FIG. 11 illustrates one more prior-art example of the edge emphasizing circuit 908 shown in FIG. 9. In this example, R and B signals and not just the G signal are used as edge detection signals. The Yg signal (G signal) obtained by adaptive interpolation in the adaptive interpolation circuit 904 is input to bandpass filters 1101 to 1103 via the luminance signal creating circuit 907. A bandpass filter circuit 1104 for detecting high-frequency components in the horizontal direction (HBPF) detects an edge signal in the horizontal direction of Yg. The edge signal is subjected to amplitude adjustment in a PP gain circuit 1113 and noise components are removed by a BC circuit 1122. Similarly, edge signals in the vertical direction (VBPF) and diagonal direction (DBPF) of Yg also are detected. The edge signals are added by adders 1130, 1131 and a gain adjustment is applied in a gain circuit 1134 to obtain a signal Ygapc.
Further, Yr (R signal) and Yb (B signal) that are output from the adaptive interpolation circuit 904 also are subjected to similar processing to obtain edge signals Yrapc, Ybapc, respectively. The Ygapc, Yrapc, Ybapc signals are added by an adder 1137 to obtain edge signal Yapc. An adder 1138 adds the edge signal Yapc and the main luminance signal Ysig [see Equation (1) above), which has been generated by the luminance signal creating circuit 907, and outputs the result.
With the example of the prior art shown in FIG. 10, however, only the G signal is used in edge detection. Consequently, a problem which arises is that a subject having a color possessing no spectral sensitivity to G (the green component) will appear significantly indistinct. (A boundary portion when a subject having the colors red and black is imaged shall be referred to as a “red-black edge” below.) Further, in the example of FIG. 11, edge signals detected from an RGB signal are further included irrespective of the color of the subject. Consequently, with the edge emphasizing processing shown in FIG. 11, a subject of the color green, for example, appears significantly indistinct in comparison with edge emphasis from the G signal alone (the edge emphasis processing of FIG. 10). Furthermore, as shown in FIG. 12, the R, B spectral sensitivity of a primary-colors image sensing device currently in use generally is fairly low in comparison with that of the G signal, as shown in FIG. 12. The R, B signals, therefore, are raised in gain by white balance gain and noise components also are enlarged. As a result, the edge emphasis signals obtained from the R, B signals include many noise components. Though blurring of the red-black edge is improved upon in comparison with the prior-art example of FIG. 10, noise increases and image quality declines.